Technologies used in conversion between alternating current (ac) and direct current (dc) have advanced rapidly over the past decades, as has the use of dc as a means of transporting large amounts of power. Dc transports power more efficiently than ac and unlike ac, is able to do so at a controlled level, e.g. at a level corresponding to the maximum thermal capability of the conductors used.
The number of high voltage dc transmission projects has grown in distance, voltage level and power rating. Furthermore the need for dc-to-dc transformation has been limited since, irrespective of the dc transmission voltage, ac transformers are included in both sending and receiving terminals so that conversion to a voltage convenient to the local system is quite straight forward. However ongoing evolution of the world's electric supply system gives rise to at least four general applications where direct dc-to-dc transformation can have an important role.                1. Both in Europe and North America, system planners foresee the need for an extensive, very high voltage dc network to serve as an overlay to the existing ac transmission system. However in addition to the connection of such a grid to key points in that ac system, an economic dc-to-dc transformer will allow both interconnection of that grid to a number of major existing dc transmission lines and transformation of different dc voltages among dc lines comprising a dc grid, thereby providing major additional economic benefits to consumers of electricity.        2. In context of a dc grid the ac-to-dc converters that connect ac systems to the dc grid will have limited control of power through the various dc transmission lines comprising the grid. Dc-to-dc transformers placed strategically in the dc grid will provide a desired dc power flow through the transmission lines of the dc grid where it is not possible to accomplish with the ac-to-dc converters        3. The electrical architecture of wind farms is relatively complex and, in some systems, makes use of ac-to-dc conversion at the wind turbine site to accommodate the asynchronous nature of the ac voltage generated by variable speed blades. Rather than reconvert that dc to synchronous ac, as is now the practice, direct transformation of that dc energy to a high dc level for aggregation within a wind farm, and to a yet higher dc voltage for transmission to a load center, could affect major cost reductions in the aggregation of power at a central point on the wind farm as well as transmission of the collected power to a convenient point on the ac receiving system.        4. Household, commercial, and industrial loads are increasingly either dependent on dc or, in some cases, could operate more efficiently if converted to dc. Charging load for electric automobiles, projected to grow very rapidly as a percentage of total electrical load, is a prime example. This realization has steadily increased the interest in dc distribution systems whereby dc loads, rather than depending on either small scale plug-in or internal ac-to-dc converters, could be served directly from existing distributions poles or cable routes.        